The use of impact type particle separators to remove solid material entrained in a gas is well known. Typical examples of such particle separators are illustrated in U.S. Pat. No. 2,083,764 to Weisgerber, U.S. Pat. No. 2,163,600 to How, U.S. Pat. No. 3,759,014 to Van Dyken, II et al., U.S. Pat. No. 4,253,425 to Gamble, et al., and U.S. Pat. No. 4,717,404 to Fore.
Particle separators for CFB reactors or combustors can be categorized as being either external or internal. External type particle separators are located outside the reactor or combustor enclosure; see, for example U.S. Pat. No. 4,165,717 to Reh, et al., U.S. Pat. No. 4,538,549 to Stromberg, U.S. Pat. Nos. 4,640,201 and 4,679,511 to Holmes et al., U.S. Pat. No. 4,672,918 to Engstrom, et al., and U.S. Pat. No. 4,683,840 to Morin. Internal type particle separators are located within the reactor or combustor enclosure; see, for example U.S. Pat. Nos. 4,532,871 and 4,589,352 to Van Gasselt, et al., U.S. Pat. Nos. 4,699,068, 4,708,092 and 4,732,113 to Engstrom, and U.S. Pat. No. 4,730,563 to Thornblad.
These latter internal type separators either involve baffles across the entire freeboard space that would be difficult to unclog and support or they involve an internal baffle and chute arrangement which closely resembles the external type of particle separators.
FIGS. 1-4 are schematics of known CFB boiler systems used in the production of steam for industrial process requirements and/or electric power generation. Fuel and sorbent are supplied to a bottom portion of a furnace 1 contained within enclosure walls 2, which are normally fluid cooled tubes. Air 3 for combustion and fluidization is provided to a windbox 4 and enters the furnace 1 through apertures in a distribution plate 5. Flue gas and entrained particles/solids 6 flow upwardly through the furnace 1, releasing heat to the enclosure walls 2. In most designs, additional air is supplied to the furnace 1 via overfire air supply ducts 7.
Several variations of particle separation and return to the furnace 1 are known. The FIG. 1 system has an external cyclone primary separator 8, a loop seal 9, and optional secondary collection discussed infra. The systems of FIGS. 2-4 typically provide two stages of particle separation. FIG. 2 has a first stage external impact type particle collector 10, particle storage hopper 11, and L-valve 12; FIGS. 3-4 employ in-furnace impact type particle separators or U-beams 13 and external impact type particle separators or U-beams 14. The in-furnace U-beams return their collected particles directly into the furnace 1, while the external U-beams return their collected particles into the furnace via the particle storage hopper 11 and L-valve 12, collectively referred to as a particle return system 15. An aeration port 16 supplies air for controlling the flow rate of solids or particles through the L-valve 12.
The flue gas and solids 6 pass into a convection pass 17 which contains convection heating surface 18. The convection heating surface 18 can be evaporating, economizer, or superheater as required.
In the FIG. 1 system, an air heater 19 extracts further heat from the flue gas and solids 6; solids escaping the external primary cyclone separator 8 may be collected in a secondary collector 20 or baghouse 21 for recycle 22,23 or disposal as required. Systems in FIGS. 2-4 typically use a multiclone dust collector 24 for recycle 25 or disposal as required, and air heaters 26 and baghouses 27 are also used for heat extraction and ash collection, respectively.
In CFB reactors, reacting and non-reacting solids are entrained within the reactor enclosure by the upward gas flow which carries solids to the exit at the upper portion of the reactor where the solids are separated by internal and/or external particle separators. The collected solids are returned to the bottom of the reactor commonly by means of internal or external conduits. A pressure seal device (typically a loop seal or L-valve) is needed as a part of the return conduit due to the high pressure differential between the bottom of the reactor and the particle separator outlet. The separator at the reactor exit, also called the primary separator, collects most of the circulating solids (typically from 95% to 99.5%). In many cases an additional (secondary) particle separator and associated recycle means are used to minimize the loss of circulating solids due to inefficiency of the primary separator.
U.S. Pat. No. 4,992,085 to Belin, et al discloses the internal impact type particle separator shown in FIGS. 3-4 of the present application discussed above. It is comprised of a plurality of concave impact members supported within the furnace enclosure and extending vertically in at least two rows across the furnace exit opening, with collected particles falling unobstructed and unchannelled underneath the collecting members along the enclosure wall. This separator has proven effective in increasing the average density in a CFB combustor without increasing the the flow of externally collected and recycled solids. This has been done, while providing simplicity of the separator structural arrangement, absence of clogging, and uniformity of the gas flow at the furnace exit. The latter effect is important to prevent local erosion of the enclosure walls and in-furnace heating surfaces like wingwalls caused by impingement of a high velocity gas-solids stream.
In this known embodiment, the internal impact type particle separator, comprised of two rows of impingement members, is typically used in combination with a downstream external impact type particle separator from which collected solids are returned to the furnace by an external conduit. The external impact type particle separator and associated particle return means, e.g., the particle storage hopper and L-valve, are needed since the efficiency of the internal impact type particle separator, comprised typically of two rows of impingement members, is not sufficient to prevent excessive solids carryover to the downstream convection gas pass which may cause erosion of the convection surfaces and an increase of the required capacity of the secondary particle collection/recycle equipment.
It is known that the efficiency of an impact type particle separator increases when the number of rows of impingement members increases from two to four or five. One arrangement of an internal impact type particle separator is disclosed in U.S. Pat. No. 4,891,052 to Belin, et al. However, the efficiency of the internal impact type particle separator of U.S. Pat. No. 4,891,052 cannot be improved by simply increasing the number of rows because of a) greater reentrainment of the discharged solids by gases, with the upward gas velocity increasing sharply in the direction to the center of the furnace, and b) increasing bypass gas flow through the discharge area of the impingement members.
It is apparent that a CFB reactor or combustor could be made more simple and less costly by a design which provided for entirely internal primary particle separation and return, thus eliminating the need for any external particle return means.